Bearing for wind turbine

ABSTRACT

A bearing assembly ( 12 ) for a rotating element ( 13 ) has one race ( 15 ) adapted to be fixed relative to ground ( 11 ) and to selectively be free for arcuate movement relative to ground. In a preferred embodiment a selective locking device ( 18 ) is provided for the relatively fixed/movable race ( 15 ).

TECHNICAL FIELD

This invention relates to a bearing comprising a relatively fixed racefor a rolling element, and particularly to a bearing comprising an innerrace and a concentric outer race, preferably having a plurality ofrolling elements therebetween. The rolling elements may comprise ballsor rollers, and be arranged in one or more rows. The invention is alsoapplicable to plain and tilting pad bearings. In this specificationreference to ‘race’ includes appropriate reference to ‘bush’.

BACKGROUND TO THE INVENTION

Typically such a bearing supports the rotating load of a moving elementwith respect to a ground element. One of the races, constitutes or isfixed to the moving element and the other is fixed to the groundelement, typically by an interference press fit. The weight of the loadis taken vertically on the upward facing side of the fixed race or bush,via the rolling elements where provided. Typically, a race which isfixed to the load rotates in use, so that the load is transmittedprogressively and repeatedly around the entire circumference thereof.However the race or bush which is fixed to the ground element, beingrelatively stationary, has weight taken continuously by the same upwardsfacing portion; conversely the downwards facing portion takes no weight.As a result wear of the relatively fixed race is asymmetric, and thebearing may require replacement more frequently than if wear had beendistributed around the entire circumference of the fixed race.Non-vertical loads may be transmitted to the same sector of a relativelystationary bearing, for example due to the separating forces on a pairof parallel geared shafts.

The problem of asymmetric wear is most apparent in bearings whichsupport sustained unidirectional loads and/or support high bendingforces in the vertical plane, such as hub bearings of wind turbines. Thetraditional solution is to employ larger bearings of high precision, butalso to accept that more frequent replacement may be necessary. Largebearings are difficult and expensive to manufacture, and tend also tosuffer disproportionate deterioration due to vibration compared withsmall bearings.

What is required is an arrangement capable of eliminating this kind ofasymmetric wear profile, but which is adaptable to standard types ofbearing, particularly rolling element bearings, and particularly to thefixed and moving elements which transmit loads via the bearing.

A further problem is sustained static pressurisation of bearinglubricant between the metal surfaces of the bearing. This occurs in theregion of the upper side of the fixed race of the bearing arrangementwhen it is static. Over time, hydrogen atoms may escape from hydrocarbonmolecules in the lubricant and react with the metal surfaces resultingin hydrogen embrittlement of the metal surfaces, which increases theincidence of failure.

What is required is an arrangement capable of limiting the exposure ofthe metal surfaces to prolonged contact with static pressurizedlubricant.

One solution to these problems is to provide a bearing assemblycomprising an inner race, and an outer race concentric with the innerrace, one of said races being adapted to idle in rotation on a groundelement.

This arrangement is applicable to a plain bearing, but is particularlysuitable for a rolling element bearing having rolling elements betweenthe inner and outer races.

The bearing is thus arranged so that the race of the ground element isadapted to idle on its support. The idling speed is preferableimperceptible, and may be as low as a few revolutions over the rotationand life of the bearing. Idling may be in the range 0.1 degree per dayto 0.1 revolutions per minute.

Faster idling may be permissible according to the type and size ofbearing, and kind of installation. In any event idling above 50 rpm,alternatively one tenth of hub speed, is not envisaged. Furthermore thissolution is particularly suitable for large bearings, having a shaftdiameter in excess of 250 mm.

In order to idle, the race of the ground element slides on or in theground element, rather than being an interference fit, and thecorresponding diameter of this race may be dimensioned accordingly. Thuswhere the inner race is mounted on the ground element, the innerdiameter is slightly increased; where the outer race is on the groundelement, the outer diameter is slightly reduced. Alternatively theground element may be sized as a relatively loose fit in or on theadjacent race.

The race of the ground element should not rotate at a speed sufficientto cause wear of the ground element or of the race itself, and ispreferably at least an order of magnitude less than the speed at whichsuch wear is measurable over the life of the bearing.

The idling rotation may be continuous. However intermittent motion isalso possible, and may be suitable where the bearing can tolerate afixed position of one race for a pre-determined period—for example up to5 years. Reversing and reciprocating motion is also envisaged.

It would be desirable to regulate and/or control idling in order toallow intelligent relative positioning of the races.

SUMMARY OF THE INVENTION

According to the invention there is provided a bearing assembly for arotating element, said bearing assembly comprising a race adapted to befixed relative to ground in normal use, wherein the assembly is furtheradapted to selectively free the fixed race for arcuate movement relativeto ground. Preferably the assembly comprises an inner race, and an outerrace concentric with the inner race, one of the races being adapted forfixing against rotation on a ground element, wherein said assembly isfurther adapted to selectively free the fixed race for arcuate movementon the ground element. Such movement is typically not along therotational axis.

Such an arrangement permits the relatively fixed race to be parked onthe ground element in normal use, but shifted arcuately on demand toexpose a different sector to static loads. The arrangement also permitsthe fixed race to be shifted arcuately in response to loads in otherdirections, if it is determined that such loads may lead todeterioration of the bearing surface supporting said loads.

Preferably the bearing assembly comprises an integrated locking devicemovable axially with respect to the normally fixed race from a conditionin which the normally fixed race is fixed against rotation on the groundelement, to a condition in which the normally fixed race is free forarcuate movement with respect to the ground element. Said locking devicepreferably increases the radial dimension of the bearing annulus, thatis to say a radial dimension from the inner surface of the inner race tothe outer surface of the outer race. In a preferred embodiment thelocking device comprises a wedge, preferably an annular wedge, which mayhave a wedge angle in the range 5°-89°.

The locking device may alternatively provide an axial clamping loadsufficient to restrain the normally fixed race against rotational dragforces in use.

Preferably the locking device is located at one side of the races andwithin the annular envelope defined by maximum outer radius of the outerrace and the minimum inner radius of the inner race.

In a preferred embodiment the locking device is movable in one directionby a thrust device, such as a hydraulic actuator, against the effect ofa return spring. The return spring may act directly upon the normallyfixed race. In the preferred embodiment said return spring biases thenormally fixed race to the condition where it is fixed relative to theground member. Movement of the locking device by a double acting thrustdevice is also possible, whereby the locking device is engaged anddisengaged positively rather than being moved in one direction by areturn force. A double acting hydraulic actuator may be used to move thelocking device in such an arrangement.

The locking device preferably comprises sequential mechanisms adapted tofirstly free the normally fixed race for arcuate movement relative tothe ground element and to secondly move the normally fixed racearcuately relative to the ground element. Such relative movement is in apreferred embodiment uni-directional, and may be a push or a pull.

The locking device is preferably reversible whereby relative movement ofthe normally fixed race is ceased prior to parking the fixed racerelative to the ground element.

Preferably the locking device includes a reversibly movable thrustmember having a first movement range for freeing said normally fixedrace and a second movement range for moving said normally fixed race.Said first movement range is preferably a linear progression. Saidsecond movement range is preferably also a linear progression, andadapted to be repeated on demand whilst maintaining said thrust memberat or beyond the limit of said first range of movement.

The thrust member may be an annulus centred on the rotational axis ofthe bearing assembly. Preferably the bearing assembly is a rollingelement bearing, such as a cylindrical roller bearing or taper rollerbearing.

The thrust member preferably comprises a single actuator for both firstand second stages. The thrust member may include a step mechanismwhereby repeated strokes of said actuator are cumulative andproportional to movement of said thrust member over the first movementrange.

Preferably said normally fixed race includes a one-way clutch forconnection to a ground element whereby unidirectional arcuate movementthereof is assured in response to movement of said thrust member in thesecond movement range.

Preferably the normally fixed race is the inner race, and the groundelement is a hub fitted into the inner race. The locking device ispreferably directly insertable between said inner race and hub in use.

In one embodiment, the normally fixed race includes a frusto-conicalsurface opposite the bearing track, and a circular wedge insertableaxially of the fixed race between said frusto-conical surface and theground element. Thus axial movement of the circular, annular wedge maylock and unlock the fixed race with respect to the ground element, ondemand.

Preferably a radially directed abutment of the fixed race retains thecircular wedge with respect thereto, but with clearance for movement ofsaid wedge from a locked to an unlocked condition.

In a preferred embodiment the cone angle of the wedge is the same as thecone angle of the fixed race.

Preferably the circular wedge is biased into engagement with thenormally fixed race by resilient means, such as cone spring orBelleville washer. Said spring may be housed in the clearance betweenthe wedge and an abutment of the normally fixed race, so as to bias thenormally fixed race to the parked condition.

The circular wedge may have splits, in the manner of a collet, so as toimprove the clamping load thereof. An anti-seize coating may be employedon the cone surfaces, as may an anti-friction coating.

The assembly may further include an actuator for said wedge. In oneembodiment said actuator comprises a hydraulic chamber adapted to exertan axial load on the wedge, on demand. The chamber may be circular aboutthe rotational axis of the bearing, and include an annular piston actingon said spring in use. Alternatively the actuator may be mechanical, inthe form of a cam. Preferably the actuator is an annular cam co-axialwith the circular wedge and having one or more ramps adapted to urge thewedge axially upon rotation of the cam about said axis. Said annular camis preferably indexed by a tangentially mounted thruster.

The thruster may be in the form of a hydraulic thruster having a fixedstroke and engageable with a tooth of said annular cam. Preferably aplurality of teeth are on the radially outer circumference of said cam,and said thruster is mounted relative to the ground element forengagement with said teeth.

In a preferred embodiment said annular cam includes a releasableanti-reversing latch whereby successive actuations of said hydraulicactuator engage successive teeth of said cam so as to progressivelyrotate said cam about said axis with respect to said actuator.

The teeth of the cam are preferably ratchet teeth, each tooth having asubstantially radial advancing face for engagement by said actuator.

The anti-reversing latch is preferably a relatively fixed pawl pivotableinto engagement with an advancing face on demand, but adapted to ratchetover said teeth. For example a second hydraulic thruster may be providedto urge said pawl into engagement via a resiliently compressible link.

In a preferred embodiment said cam further includes an abutmentengageable with a nudge member mounted on said normally fixed race via aone-way clutch. Thus arcuate movement of said cam under the action ofsaid hydraulic thruster causes nudging of said normally fixed race in anadvance direction relative to said wedge. Accordingly the position ofload on the fixed race is altered.

The fixed race, once nudged, may stay in the new position by virtue offrictional forces, but preferably a second one-way clutch is providedoperationally between the fixed race and the wedge in order to preventreverse movement. Successive nudges of the nudge member result insuccessive movements of the normally fixed race, and appropriatere-positioning thereof.

The nudge member is preferably in the form of a ring concentric with theaxis of rotation of the bearing.

The one-way clutch or clutches may be in the form of a rotary sprag,wrap spring, or any other convenient form.

In a preferred embodiment the cam may include ratchet teeth over part ofthe circumference only, successive ratchetings across all teeth beingsufficient to disengage said wedge and normally fixed race. Ratchetingbeyond the final tooth is not possible since the anti-reversing pawl hasno corresponding advancing face, and accordingly further successivestrokes of said hydraulic thruster are adapted to reciprocate the cam,and provide said nudging action.

Many alternative means of driving a cam ring are possible, in order tomove the normally fixed race to and from the parked condition. Forexample a hydraulic motor, typically in the form of a hydrodynamicturbine with multiple jets, may tum the cam ring via a suitable stepdown gear. One suitable gear form is a worm drive having a ratio ofgreater than 60:1 whereby reverse motion is inhibited.

Where regular arcuate movement of the normally fixed race is desired, aclock-type escapement may be used to meter rotation, for example througha fixed angle; each oscillatory stroke of the escapement thus regulatesprecisely the corresponding movement of the normally fixed race. Theescapement may be of any suitable kind, and is not limited to mechanicalarrangements. A typical mechanical escapement could comprise a pawlengageable with successive teeth of a toothed ring of the kind describedabove in relation to the fixed stroke thruster.

BRIEF DESCRIPTION OF DRAWINGS

Other features of the invention will be apparent from the followingdescription of several preferred embodiments shown by way of exampleonly in the accompanying 25 drawings in which:

FIG. 1 is a schematic axial section through a first embodiment of theinvention.

FIG. 2 is an enlarged view of the circled part of FIG. 1.

FIG. 3 is a schematic axial section through a second embodiment of theinvention.

FIG. 4 is a schematic axial section through a third embodiment of theinvention.

FIG. 5 is a perspective view of a bearing assembly according to a fourthembodiment of the invention.

FIG. 6 is a schematic axial section through the embodiment of FIG. 5.

FIG. 7 is an enlarged view of the indexing mechanism of FIG. 6.

FIG. 8 is a schematic illustration of the operation of the fourthembodiment.

FIG. 9 is an exploded view of the components of the fourth embodiment,and

FIG. 10 is a schematic arrangement of a control system suitable for theinvention.

DESCRIPTION-OF PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2 a stationary circular hub 11 has thereona rolling element bearing 12 to support a rotating member 13, which mayfor example be a rotor of a wind turbine.

The rolling element bearing comprises an outer race 14 pressed into therotating member 13, an inner race 15, and a circular array of rollers 16between the races.

In order to allow easy fitting and removal of the bearing 12 to the hub11, the inner race has a frusto-conical radially inner surface 17adapted to receive a corresponding annular fitting member 18 with afrusto-conical radially outer surface 19. This arrangement permits therolling element bearing to be placed over the hub, and locked in placeby pressing in the fitting member 18 in the direction of arrow ‘A’. Bysuitable selection of the taper angle, in a well-known manner, thefitting member will fix the bearing in place relative to the hub, yet beremovable on application of a release load opposite to the direction ofarrow ‘A’. The fitting member may be self locking, or may be retained byany suitable means, such as a circlip.

Thus it will be understood that in the locked condition of the fittingmember 18, the inner race 15 may be subjected over time to potentiallydamaging forces on the load bearing upper face, whereas in the unlockedcondition of the fitting member, the inner race is free to be movedarcuately so as to change the load bearing portion thereof.

Only slight relative movement between the inner race and the fittingmember is required to achieve freedom of movement of the inner race, andit is possible for either the fitting element or the inner race to befixed axially relative to the hub.

As shown in greater detail in FIG. 2, a circular outer housing 21 isfixed relative to the inner race 15. The outer housing may be split on adiameter and for example mounted to a suitable surface of the inner raceby a circular shoulder 22 and circumferential clamp (not shown).

A Belleville spring washer 23 acts between radially inwardly directedabutment 24 of the outer housing 21, so as to bias the fitting member toa condition in which the inner race 15 is fixed or parked relative tothe hub 11.

A circular inner housing 25 may be press-fitted to the fitting member 18and has radially outwardly extending portions which co-operate withcorresponding portions of the outer housing 21 to define a hydraulicpiston chamber 26. Suitable annular sliding seals 27, 28 close thechamber 26. The housing may alternatively be screw-threaded to thefitting member, as illustrated.

In use hydraulic oil under pressure can be admitted into the chamber 26to cause the inner housing 25 to be urged to the right (as viewed)relative to the outer housing 21. As a result the fitting member 18moves to the unlocked condition whereby the inner race 15 is able tomove circumferentially.

In order to promote movement of the fitting member 18, and to facilitatearcuate movement of the inner race 15, hydraulic oil under pressure mayalso be admitted to the space between surfaces 17 and 19 to form ahydrostatic bearing. Suitable sliding seals may be provided to confinethis oil, of which one is shown at 29.

The hydraulic oil is preferably the same as that used for lubrication ofthe rolling element bearing.

A second embodiment is illustrated in FIG. 3, in which parts common tothe first embodiment carry the same reference numerals.

A hydraulic oil feed 31 has passages through shaft 11 to feed both thefrusto-conical space 32 between the inner race 15 and the fitting member18, and the piston chamber 26. The space 32 is open to the chamber 33housing the Belleville spring 23, but is closed at the opposite end byseal 34. The spring chamber 33 has a vent 35.

An annular piston 36 is keyed to the fitting member 18 against relativerotation, but can slide axially of the shaft. In the unenergizedcondition, the piston chamber 26 is unpressurized and the Bellevillespring 23 urges the fitting member to the left (as viewed) into lockingengagement. The piston 36 is urged to the right by the spring 23 untilabutting against an inturned lip 37 of the outer housing 21. This lip 37also prevents the inner race 15 from sliding off the fitting member tothe left as viewed.

Clearances are somewhat exaggerated in FIG. 3 to demonstrate theprinciple of operation, but in the engaged condition the inner race 15is parked against arcuate movement with respect to the shaft 11, thecentreline 20 of which is also illustrated.

Upon pressurization of the oil feed 31, the piston 36 moves to the leftaway from abutment with the lip 37 (as illustrated) and the fittingmember 18 is free to float relatively to the right so as to unlock theinner race 15.

In FIG. 3, the piston chamber is defined by an annular housing 38 whichis held in place by a locking ring 39, but is free to float on anannular extension 40 of the fitting member when unpressurized. Numeroushydraulic seals confine the hydraulic fluid, as will be readilyunderstood. The vent 35 permits fluid pressure to drain from theassembly, and is of a size selected to maintain sufficient operationalpressure in the chamber 26 when required.

Alternatively a suitable restrictor may be placed in the fluid supplyline to the space 32.

A third embodiment is illustrated in FIG. 4 and comprises a two-partfitting member comprising a sleeve 41 fixed to the shaft 11 and havingan annular frusto-concial ramp face 42 at one end, and an axiallymovable annular frusto-conical wedge 43 at the other end.

The ramp face 42 and wedge 43 confine an inner race 15A which hasmatching frusto-concial faces on the radially inner side, and can moverelatively axially together to lock the inner race 15A to the sleeve 41,thereby to prevent relative rotation of the inner race relative to theshaft 11.

In the embodiment of FIG. 4, an arcuate cam 44 is located about thesleeve 41 and is movable circumferentially to load or unload the wedge43, as will be further explained. An abutment 45 is provided for the cam44, and is retained by a nut 46 or the like. A retainer 47 for theBelleville spring 23 is keyed to the sleeve 41, and suitable rollers 48are provided between the relatively movable elements, as illustrated.Three or more equispaced cams may be provided to evenly distribute theaxial load on the wedge.

The embodiment of FIG. 4 has the advantage that the inner race islocated axially of the shaft 11, and is not subject to axial loads fromthe fitting member. In FIG. 4 such axial loads are resisted by the rampface 32, and thus not transmitted to the outer race 14 as in theembodiment of FIG. 3. Furthermore, the inner race is maintainedcentrally when unparked.

FIGS. 5-8 illustrate a fourth embodiment incorporating a mechanicalrelease for the fitting member. As with other embodiments commonreference numerals are used for parts having the same function. Thisembodiment provides a self-contained bearing assembly adapted forfitting to a suitable hub, and in which the actuation mechanism is atone side and within the annulus defined by the hub 11 and the externaldiameter of the outer race 14.

The fitting member 18 is coupled against rotation with respect to thehub 11 by a key 51.

A first hydraulic actuator 52 is connected to a pawl 53 which engagesexternal teeth of a cam ring 54. One stroke of the first actuatorindexes the cam ring by one tooth in the direction of arrow B. Theactuator is responsive to pressurization from a suitable hydraulicsource and control system. Removal of pressure causes the pawl to bewithdrawn by a light spring. A leaf spring biases the pawl intoengagement with the cam ring as it advances from the retractedcondition.

An anti-reversing lever 55 is pivoted against the centre and has a tooth56 at one end which is engageable with the teeth of the cam ring 54. Alight coil spring 57 pivots the tooth 56 out of engagement, and isopposed by a second hydraulic actuator 58 which can be pressurized toapply a load via a spring (see FIG. 7), so as to place the tooth 56 intoengagement with the cam ring. The teeth of the cam ring 54 are ratchetteeth, and pass over tooth 56 in the direction of arrow B when the toothis biased inwards by the second actuator 58.

In use, pressurization of first actuator 52 and second actuator 58causes the cam ring to be advanced in the direction of arrow B, and tobe retained in the advanced condition by the tooth 56 ratcheting over anadjacent tooth of the cam ring.

Upon release of pressure in the first actuator 52, the cam ring 54remains in the advanced condition. A further pressurization of the firstactuator causes the cam ring to advance another step, and so on.Pressurization may be by way of a square wave electrical signal.

Thus so long as the second actuator 58 remains pressurized, the cam ring54 can be advanced by successive pressure pulses to the first actuator52.

As best seen in FIG. 7, a Belleville spring 23 acts between the fittingmember 18 and a backing ring 59, which itself abuts a lip 37 of an outerhousing 21 of the inner race 15. Thus as illustrated in FIG. 7, thespring 23 forces the fitting member into engagement with the inner race,which is consequently locked against rotation relative to the shaft 11.

The cam ring 54 includes one or more cam surfaces adapted to urge thebacking ring 59 to the left as viewed. Upon being urged to the left, thebacking ring lifts off the lip 37, and the fitting member 18 is free tofloat to the right, thereby unlocking the inner race 15, and permittingarcuate movement relative to the hub. Consequently it will be understoodthat successive pressure pulses to the first actuator 52 can rotate thecam ring 54 through angle ‘C’ (FIG. 8) which is sufficient to releasethe fitting member and hence the inner race.

FIG. 7 also illustrates an annular travel stop 61 to prevent excessivemovement of the fitting member with respect to the inner race. Oil fromthe hydrostatic bearing formed between the fitting member and inner raceis held back by this travel stop when the inner race is released.

The fourth embodiment also provides a means of moving the inner racearcuately, when unlocked.

The number of teeth on the cam ring 54 is limited, and the last tooth isfollowed by a relatively long ramp face 62 beyond which no recess forthe tooth 56 is provided. Consequently when the first actuator has madesufficient strokes for the cam ring 54 to present the last ratchet toothto the lever 55, further strokes merely allow the cam ring to oscillateback and forth through angle ‘D’ (FIG. 8).

However, an adjacent annulus 63 of the cam ring has a radial abutment 64which is arranged to come into contact with a radial abutment of a nudgering 65, so that oscillation of the cam ring 54 causes oscillation ofthe nudge ring 65 against the effect of a light return spring 66.

The nudge ring 65 is mounted via a conventional one-way clutch, or sprag65 a to an annulus of the inner race 15, such that oscillation of thenudge ring results in repeated one-way indexing of the inner race. Inthis way the inner race may be repositioned relative to the hub, so asto place a different sector in the position where vertical loads arereacted; successive nudges are indicated at ‘E’ (FIG. 8).

The inner race 14 is mounted via a second one-way clutch 70 grounded onthe hub, so as not to reverse back to the position prior to nudging.

A collar 67 and nut 68 retain the components to the fitting member 18.The collar 67 is keyed to the fitting member as illustrated.

FIG. 9 is an exploded view of the components of the assembly of FIGS.5-8.

The fitting member 18 includes rectangular recesses 71 on thefrusta-conical surface thereof, which comprise hydrostatic bearing padsadapted to assist in disengaging the fitting member from the inner race.

These pads are supplied with hydraulic oil under pressure, as describedin relation to the first embodiment.

In order to permit detection of the number of strokes of the firstactuator, and hence the position of the cam ring 54 and inner race 15, aseries of axially extending projections 72 of the cam ring may beadapted to ring a ‘bell’ 73 by simple contact therewith. For examplesuch projections may have the same pitch as the ratchet teeth. The‘ring’ may be different according to the position of the cam ring, forexample by using a projection of different profile. ‘Ringing’ of thebell may be detected by suitable accelerometers or the like and providean input to a control system which is now described with reference toFIG. 10.

A gearbox 80 for the driveline or gearbox of a wind turbine contains arotor bearing 81 of the kind described by reference to FIGS. 1-9.Hydraulic oil within the gearbox is circulated to an actuator for afitting member of the bearing via a pump 82 and valve 83.

The bearing 81 incorporates a tum function 84 for the inner race, and arelease/hold function 85. The hydraulic signal to the release/holdfunction is via an accumulator 86 having a restricted vent 87.

In use the valve 83 is opened repeatedly to feed the functions 84 and85. The fitting member is released, and the inner race turned in themanner described with reference to FIGS. 5-7. The accumulator 86 ensuresthat pressure is maintained in the second actuator so as to avoidreversing of the cam ring 54.

Upon final closure of the valve 83, for example when the inner race isin a new position, the hydraulic pressure drains via vent 87 so that thecam ring returns to the start condition whereby the fitting member isre-engaged.

It will be understood that in the event of a failure of the pump 82 orloss of hydraulic pressure due to a leak or the like, the fitting memberwill mechanically re-engage under the action of the Belleville spring.

A percussive signal generator 88, such as a bell, emits a characteristicrepresentative of a specific angular rotation of the cam ring, and thissignal is detected by a suitable sensor 89.

A bearing control unit 90 includes modules for actuation strategy 91,bearing condition calculation 92, and bearing position calculation 93.

A wind turbine control unit 95 may include a module for bearingcondition calculation 96 in place of module 92.

In use the wind turbine control unit 95 receives typical inputs of forexample speed, torque, temperature, state of emergency brake, percentagepower de-rating and the like in order to predict the condition of thehub bearing. Thus an algorithm may indicate at what stage rotation ofthe relatively fixed race is desirable, said algorithm being based uponaccumulated knowledge from several bearing installations.

Upon determining that movement of the fixed race is desired, the controlunit 95 will output to the bearing control unit 90 a signal 101 giving,for example, an instruction to move the fixed bearing by a predeterminedarcuate amount, say 20°.

The bearing control unit will send successive signals 102 to open thevalve 83 in order to release the fixed bearing race, and turn it thoughthe desired angle. Feedback is provided by the vibration sensor signal103.

The actuation strategy module 91 controls the valve 83 in a desiredmanner, and the bearing condition calculation module 92 (or 96)calculates and records the condition of the fixed bearing race.

A typical strategy for determining position of a fixed bearing race may,for example, to use a look-up table in conjunction with real-timeoperational information to rotate the race by 20° every 5000 hours. Aprime number rotation sequence will avoid the risk of parking the racein the same position with the consequent risk of further localizeddeterioration.

A strategy may optimize bearing position over a fixed life, say 20years, so as to distribute potential deterioration around thecircumference of the fixed race.

Furthermore a strategy may use a determination of a damaged race sectorto avoid further parking in the vicinity of that sector. Such a strategyis useful where nonstandard damage is experienced, for example due to amanufacturing defect.

In the described embodiments, the hub and inner race are assumed to bestationary in normal operation. However it will be understood that outerrace and housing thereof could be stationary, and the inner race turnwith for example a rotor connector thereto.

In such circumstances, idling of the outer race is required in order toavoid deterioration of the sector bearing the vertical load.

The invention requires certain components to be attached together, andwhich may be of dissimilar materials, such as the inner bearing race 15and housing 21. Any suitable attachment method may be used, for exampleheat or friction welding, threaded engagement or dog engagement.

1. A bearing assembly for rotatably supporting a rotating elementrelative to ground and comprising: a race adapted to be fixed relativeto ground and adapted to be freed relative to ground in arcuatemovement; a mechanism adapted to selectively free the race which isfixed relative to ground for arcuate movement relative to ground; and anactuator adapted to move the race which is freed relative to groundarcuately.
 2. A bearing assembly according to claim 1, wherein the racecomprises an inner race, and an outer race concentric with the innerrace, one of the inner race or outer race being adapted for fixingagainst rotation on a ground element.
 3. A bearing assembly according toclaim 2, further including a locking device adapted to engage one of theinner race or the outer race on demand.
 4. A bearing assembly accordingto claim 3, wherein the locking device is adapted to engage a peripheralsurface of one of the inner and outer races.
 5. A bearing assemblyaccording to claim 4, wherein the locking device is adapted to engage acircumferential surface of one of the inner and outer races.
 6. Abearing assembly according to claim 5, wherein the ground is a shaft andthe locking device is an axially movable wedge.
 7. A bearing assemblyaccording to claim 1, wherein the actuator comprises a cam ringrotatable about a rotational axis of the bearing assembly.
 8. A bearingassembly according to claim 1, wherein the bearing assembly is confinedwithin a tubular envelope defined by an outer diameter of the outer raceand an inner diameter of the inner race.
 9. A bearing assembly,comprising: an inner race and an outer race, at least one of the innerrace and outer race being selectively rotatable or locked againstrotation relative to each other or a ground; a locking device toselectively rotationally lock or unlock at least one of the inner raceand outer race relative to each other or ground; and an actuator torotate an unlocked race.
 10. A bearing assembly according to claim 9,wherein the inner and outer races are concentric.
 11. A bearing assemblyaccording to claim 10, wherein the locking device is adapted to engageone of the races on demand.
 12. A bearing assembly according to claim11, wherein the locking device is adapted to engage a peripheral surfaceof one of the races.
 13. A bearing assembly according to claim 12,wherein the locking device is adapted to engage a circumferentialsurface of one of the races.
 14. A bearing assembly according to claim13, wherein the locking device is an axially movable wedge.
 15. Abearing assembly according to claim 9, wherein the actuator comprises acam ring rotatable about a rotational axis of the bearing assembly. 16.A bearing assembly according to claim 9, wherein the assembly isconfined within a tubular envelope defined by an outer diameter of theouter race and an inner diameter of the inner race.